I'm using a 300W PSU to run an A64 3000+, 1GB RAM, two 120GB HDDs, separate DVD writer and CD-writer, FX5200 gfx card, DV/AV video capture card, TV tuner. The PSU is an iBox unit that came with the cabinet and I tweaked it a bit for reliability.

I'm not arguing against anyone recommending a more powerful PSU - it's always good to have a healthy margin of power, but I just wanted to give a balanced point of view. This rig has been running for about a month now without any crashes or instability.